Many attempts have been made to simulate natural daylight by artificial means. It has been claimed, with some justification, that natural daylight is the preferred lighted environment. Thus, for example, in form 00112 8809L 150M (1990), the Duro-Test Corporation (of 9 Law Drive, Fairfield, N.J.) states that a good simulation of natural daylight ". . . encourages people to perform as never before because it promotes good vision . . . People see better and work better . . ." Thus, in form 0090 (1988), the Duro-Test Corporation states that light which ". . . simulates natural daylight. . . " is ". . . the perfect interior lighted environment . . . "
One of the recognized experts on the use of color and its effects upon people, Mr. Carlto Wagner, has published a book entitled "Beyond Beautiful" (The Wagner Institute for Color Research, Santa Barbara, Calif., 1990). At page 38 of this book, he recommends that, when women are evaluating the colors of objects, they should "arrange them . . . under a good light source (daylight or bright incandescent lighting is best . . . ").
The standard spectrum for daylight is referred to in D. L. MacAdam's "Color Measuement: Theme and Variations" (Springer-Verlag, N.Y., 1981). In this book, the author refers to the CIE's D.sub.65 illuminant.
It is desirable to be able to simulate other daylight spectra, besides the D.sub.65 spectra. Thus, as is well known to those skilled in the art, the spectra of daylight will vary depending upon the daylight, upon atmospheric conditions, and solar altitude; see, e.g., S. T. Henderson's "Daylight and Its Spectrum," Second Edition (John Wiley & Sons, New York, 1977), the disclosure of which is hereby incorporated by reference into this specification.
There are devices known to the prior art which are capable of generating different spectral distributions; see, e.g., the device disclosed in German patent 1,744,824 of Karl. However, as the spectra generated by these devices change, the intensity of such spectra change.
For many applications, it is desirable that a sample be viewed under different spectral distributions at a substantially constant irradiance. Thus, for example, A.S.T.M. Standard D-1729-60T, entitled "Visual Evaluation of Color Differences in Opaque Materials," is often used for color matching of painted surfaces. The A.S.T.M. Standard requires that a specified opaque surface is to be viewed under different spectral conditions and that the irradiance of each of such condition be substantially constant. Thus, in section 5.1.2 of the Standard, it is provided that "For critical evaluation of color differences of materials of medium lightness, the illumination at the center of the viewed area shall be 100 to 125 foot-candles . . . In viewing very light materials, the illumination may be reduced to as low as 50 foot candles . . . and for very dark materials may be increased to as much as 200 foot candles . . . "
The device of the Karl patent is not capable of producing different spectral distributions at a substantially constant level of irradiance.
The prior art repeatedly suggests that, although different spectral distributions may be produced from a single light source and a multiplicity of filters, such distributions will have substantially different irradiances. Thus the "Kodak Color Darkroom Dataguide" (published by the Photographic Products Group, Eastman Kodak Company, Rochester, N.Y. 14650) discusses the adjustments one must make when developing prints using a single light source and different filters. At page 48 of this dataguide, in a paragraph entitled "Exposure Adjustments for Filter-Pack Changes," it is taught that "The overall density of a print is affected by changes in the number and density of the filters in the filter pack . . . For example, if you add 10M to the filter pack, increase exposure by 10 percent." The filter pack referred to in this quotation is mentioned at page 39 of the dataguide, in a paragraph entitled "Kodak Color Printing (CP) Filters" which states that "You place these acetate filters between the light source and the negative . . . When you change the filter pack, you can calculate exposure adjustments from the filter factors for the filters you add or remove."
Not everyone who uses darkroom apparatus has both the expertise, the time, and the patience to calculate correction factors for the differing spectral distributions which must be used to correctly expose a photographic print. Not everyone who uses color matching apparatus to view objects realizes the importance of viewing the objects with spectra which have substantially constant irradiances.
To the best of applicant's knowledge, the prior art did not provide an inexpensive, relatively uncomplicated apparatus which could automatically make the adjustments suggested in the Eastman Kodak dataguide. However, some extraordinarily complicated and expensive apparatuses have been designed which are suitable for similar purposes.
Thus, by way of illustration the Eastman Kodak Company has obtained several patents on complex electro-optical mechanical devices for exposing photographic products which require a substantial amount of sophisticated equipment, expertise, time, money, and patience to use.
Thus, for example, applicant's U.S. Pat. No. 4,922,089 (assigned to Eastman Kodak Company) describes an illumination source which is comprised of a spectroradiometer, a stepping motor, and a variable power supply. This device, which costs in excess of $300,000, must be used in conjunction with a sophisticated computer to control and continually adjust the elements of the device.
It is an object of this invention to provide an inexpensive device which can produce at least two different spectral distributions, each of which have substantially constant irradiance.
It is an object of this invention to provide a device which can be operated with only one control means to provide different spectral distributions with substantially constant irradiance from a light source.
It is an object of this invention to provide an adjustable filter means which, at different levels of adjustment, will provide different spectral distributions, each of which have substantially constant irradiance.
It is an object of this invention to provide a device which can provide different spectral distributions and substantially constant irradiance levels but at different illumination angles.
It is an object of this invention to provide a device which can provide spectra with different color temperatures but with substantially constant irradiance levels.
It is an object of this invention to provide a device which can provide spectra with a color temperature which is either greater than or less than the color temperature of the light source used in the device.
It is an object of this invention to provide a device comprised of an ultraviolet light source which can provide different spectral distributions and/or spectra with different color temperatures, wherein each of said spectra or spectral distributions have substantially the same irradiance level.
It is an object of this invention to provide a device which can simulate all possible daylight conditions, each with the same irradiance.
It is an object of this invention to accomplish one or more of the aforementioned objects with a relatively simple and inexpensive device which contains only one mechanical control means.
It is an object of this invention to accomplish one or more of the aforementioned objects with a device which contains an ultraviolet light source.
It is an object of this invention to accomplish one or more of the aforementioned objects with a device whose spectral output can be viewed at different angles of illumination.
It is an object of this invention to accomplish one or more of the aforementioned objects with a device equipped with means of shading ambient light from the device's viewing surface.